At least nine people were killed and 20 were injured when an explosion tore through a train carriage in a St. Petersburg metro tunnel on Monday, and Interfax news agency said the blast may have been caused by an explosive device hidden in a briefcase.
The agency, quoting unnamed sources, said surveillance cameras had captured images of what it called the organizers of the explosion, which hit St Petersburg as President Vladimir Putin was visiting the city.
Russia's National Anti-Terrorist Committee said another explosive device had been found at a different metro station, but it had been made safe.
Putin, in another part of the city for a meeting with Belarus's leader, was initially cautious. He said he was considering all possible causes, including terrorism.
Ambulances and fire engines descended on the concrete-and-glass Sennaya Ploshchad metro station. A helicopter hovered overhead as crowds gathered to observe rescue operations.
"I appeal to you citizens of St. Petersburg and guests of our city to be alert, attentive and cautious and to behave in a responsible matter in light of events," St Petersburg Governor Georgy Poltavchenko said in an address.
An attack on Russia's old imperial capital would have symbolic force for any militant group, notably Chechen secessionists and Islamic State, which is now fighting Russian forces in Syria. Chechen militant attacks in the past have largely focused on Moscow, including an attack on an airport, a theater and in 2010 a metro train.
Video showed injured people lying bleeding on a platform, some being treated by emergency services and fellow passengers. Others ran away from the platform amid clouds of smoke, some screaming or holding their hands to their faces.
A huge hole was blown open in the side of a carriage with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage.
Russian TV said many had suffered lacerations from glass shards and metal, the force of the explosion maximized by the confines of the carriage and the tunnel.
“I saw a lot of smoke, a crowd making its way to the escalators, people with blood and other people's insides on their clothes, bloody faces. Many were crying,” St Petersburg resident Leonid Chaika, who said he was at the station where the blast happened, told Reuters by phone.
ALL STATIONS CLOSED
St. Petersburg emergency services at first said that there had been two explosions. But a source in the emergency services later said that there had been only one but that the explosion had occurred in a tunnel between stations.
The blast occurred at 2.40 p.m., well shy of the evening rush hour.
Authorities closed all St. Petersburg metro stations. The Moscow metro said it was taking unspecified additional security measures in case of an attack there.
Russia has been on particular alert against Chechen rebels returning from Syria, where they have fought alongside Islamic State, and wary of any attempts to resume attacks that dogged the country several years ago.
At least 38 people were killed in 2010 when two female suicide bombers detonated bombs on packed Moscow metro trains.
Over 330 people, half of them children, were killed in 2004 when police stormed a school in southern Russia after a hostage taking by Islamist militants. In 2002, 120 hostages were killed when police stormed a Moscow theater to end another hostage-taking.
Putin, as prime minister, launched a 1999 campaign to crush a separatist government in the Muslim southern region of Chechnya, and as president continued a hard line in suppressing rebellion.